The present invention relates to a drum of a videocassette recorder, and particularly to a device for variably adjusting a moment of inertia of a drum of a videocassette recorder, in order to obtain an optium frame condition of the recorder.
Generally, one of important factors in designing a videocassette recorder (hereinafter, referred to as a VCR) is to determine a moment of inertia of rotating part of a drum of the recorder. A video head attached to the drum is periodically subjected to an impact pulse every 1/60 second in the case of NTSC mode and every 1/50 second in the case of PAL mode. In designing the rotating part of the drum, therefore, an inherent frequency of torsional vibration of the drum has to be determined such that it does not coincide with a frequency of an integral multiple of 60 Hz in the NTSC mode or 50 Hz in the PAL mode, and that the effect of high frequency of the impact pulse is minimized. The inherent frequency of torsional vibration of the drum is dependent upon an inertia moment of the rotating part of the drum. Eventually, determining the inertia moment of the drum is an important design factor.
Generally, the rotating part of the drum include a flange carrying an upper drum supporting a video head, said flange disposed at the upper portion of a rotating shaft of the drum, a motor driving said rotating shaft, and a ring collar connecting said motor and said rotating shaft, said motor and ring collar disposed at the lower portion of said rotating shaft. Such inherent frequency of the torsional vibration of the drum, denoted by "fn", is expressed by the following equation. ##EQU1## wherein, I.sub.1 : a moment of inertia of the upper rotary drum and the flange,
I.sub.2 : a moment of inertia of the rotor of the motor and the ring collar, and PA1 K : a constant determined by the rotary shaft and expressed by the following equation. EQU K=.pi.Gd.sup.4 /32 l (2) PA1 G: a modulus of shear, PA1 d: a diameter of the rotating shaft, and PA1 l: an effective length of the rotating shaft. PA1 Iio: the inertia moment of the inertia mass itself added for the n-th time, PA1 Mi: the mass added for the n-th time, and PA1 Ri: the distance of the mass added for the n-th time from the center of the rotating shaft.
wherein,
As apparent from the equation (1), the inherent frequency fn of the drum is fixed, if the moments of inertia I.sub.1 and I.sub.2 are determined. In the prior art, accordingly, it is impossible to check or test the frame condition of the VCR variable depending upon the inherent frequency of the drum, that is, the inertia moment of the rotating part of the drum, unless a new drum is used in order to vary the moment of inertia.